<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>全屏div</title>
<style>
html,body {
    margin:0;
    height:100%;
}
#test {
    width:100%;
    height:100%;
    background-color:#000;
    position:absolute;
    top:0;
    left:0;
    z-index:2;
    opacity:0.3;
    /*兼容IE8及以下版本浏览器*/
    filter: alpha(opacity=30);
    display:none;
}

#log_window {
    width:200px;
    height:200px;
    background-color:#FF0;    
    margin: auto;
    position: absolute;
    z-index:3;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    display:none;
}
</style>
<script>
function shield(){
    var s = document.getElementById("test");
    s.style.display = "block";
    
    var l = document.getElementById("log_window");
    l.style.display = "block";
}

function cancel_shield(){
    var s = document.getElementById("test");
    s.style.display = "none";
    
    var l = document.getElementById("log_window");
    l.style.display = "none";
}
</script>
</head>

<body>
<a href="javascript:shield()">打开遮罩</a>
<div id="test"></div>
<div id="log_window">
<a href="javascript:cancel_shield()">关闭</a>
</div>
</body>
</html>